1. Field of the Invention
The present invention relates to a film resistor embedded in a multi-layer circuit board and manufacturing method thereof, and more particularly, to a film resistor embedded in a multi-layer circuit board and the manufacturing method thereof that can avoid untrimmed edges of resistive materials during coating, and can improve the stability of the resistance when the film resistor is operated in high frequency.
2. Description of Related Art
Resistors are widely applied in circuits such as a current limiting circuit, a voltage regulating circuit and a high-frequency terminal impedance control circuit. In general, most miniature resistors are welded with the surface mounting technique (SMT) process. Though the resistor becomes increasingly miniature, it has to be placed on the surface of the multi-layer substrate, which increases the physical surface area and height of the circuit. To embed the resistor in the circuit board, many material developers have developed resistors coatings and applied them in various electronic circuits in recent years. However, the coated resistors often have untrimmed edges during screen printing, which causes the resistance drift, such that the resistance of the resistors after coating needs to be finely tuned with high-precision laser tools or sand-blasting grinders, which is time consuming and needs additional equipment. Furthermore, the resistance of resistors used in high-frequency circuits has to be stable and not change with the frequency.
After the normal commercial thick film resistor (TFRs) ink is coated on the substrate, the resistance thereof has the variation of about ±20% due to the unevenness in geometric structure. To obtain the precise resistance, TFR has to be trimmed with the following two methods after being sintered: (1) sand blasting grinding, and (2) laser trimming. However, the aforementioned trimming methods of the resistors are only applicable to the trimming of the coated resistors on the surface of the substrate, and extra processing equipment and cost are needed to obtain high-precision coated resistors. As for various resistors used in high-frequency applications, the resistance drift still occurs with the change of the frequency. Therefore, it is still necessary to improve the original resistor structure, so as to obtain stable resistance performance.
U.S. Pat. No. 5,990,421 entitled “Built in Board Resistors” discloses a built-in resistor as shown in FIG. 1. This patent defines the coplanar power plane or ground plane on the surface conductive layer on the multi-layer circuit board 16, such as the top conductive plane 22 or the bottom conductive plane 20 as shown, and coats resistive material 32 on the surface to form the resistor. The periphery of the coating area of the resistive material is the power plane or the ground plane, so as to form one electrode of the dual-electrode resistor. The other electrode of the dual-electrode resistor is formed by connecting the interior of the coating area of the resistive material and a signal line 26 of the inner layer through a via 28. The two electrodes are isolated with an isolation area 30 having a through hole.
In addition, U.S. Pat. No. 5,994,997 entitled “Thick-Film Resistor having Concentric Terminals and Method Therefor”, as shown in FIGS. 2A and 2B, discloses a structure similar to that of the previous patent (U.S. Pat. No. 5,990,421). FIG. 2A is a top view of a film resistor 200, and FIG. 2B is a side view of the film resistor 200. The resistive material layer of the resistor is 210, and the electrodes of the resistor are respectively an inner electrode 212 and an outer electrode 214. The inner electrode 212 is connected to a wire 220 through a bore 216, and is connected to a layer 230 as the outer electrode 218 through a bore 222. The structure of the patent mainly involves the resistor structure of the concentric ring electrode.
In addition, U.S. Pat. No. 6,284,982 entitled “Method and Component for Forming an Embedded Resistor in a Multi-Layer Printed Circuit” is shown in FIGS. 3A and 3B. The patent includes two major structures. One major structure is an inner core 300 composed by the multi-layer printed circuit board structure, and many leads or connectors 310 interconnecting with each other are provided in the inner core 300, which are isolated by a media layer 312 and is formed by using laminating structures 320. The other major structure is a film substrate including a polymeric film 330 and an adhesive layer 340. The main application of the film substrate 330 is designing the film resistor. Resistive material 344 is coated in an area 342, and the resistive electrode is led out from a through hole 346, and is connected to any position through a line 360 of a metal layer 350. However, it is only disclosed in this patent that the resistive material 344 is coated in the area 342, and the resistive electrode is led out from the through hole 346 at one end of said discrete area. This patent doesn't illustrate the resistive material, the resistive electrodes and the structure, and the resistive substrate is only limited to the structure of the aforementioned film substrate 330 and 340; therefore, the embedded resistor cannot be used in the required layer of the multi-layer substrate structure.